Intermediate layers for electrode fabrication

ABSTRACT

Provided are novel electrodes for use in lithium ion batteries. An electrode includes one or more intermediate layers positioned between a substrate and an electrochemically active material. Intermediate layers may be made from chromium, titanium, tantalum, tungsten, nickel, molybdenum, lithium, as well as other materials and their combinations. An intermediate layer may protect the substrate, help to redistribute catalyst during deposition of the electrochemically active material, improve adhesion between the active material and substrate, and other purposes. In certain embodiments, an active material includes one or more high capacity active materials, such as silicon, tin, and germanium. These materials tend to swell during cycling and may loose mechanical and/or electrical connection to the substrate. A flexible intermediate layer may compensate for swelling and provide a robust adhesion interface. Provided also are novel methods of fabricating electrodes containing one or more intermediate layers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/260,297, filed Nov. 11, 2009, entitled “INTERMEDIATE LAYERS FORELECTRODE FABRICATION,” which is incorporated herein by reference in itsentirety for all purposes.

BACKGROUND

The demand for high capacity rechargeable batteries is strong. Manyareas of application, such as aerospace, medical devices, portableelectronics, and automotive, require high gravimetric and/or volumetriccapacity cells. Lithium ion battery technology represents a significantimprovement in this regard. However, to date, application of thistechnology has been primarily limited to graphite electrodes, andgraphite has a theoretical capacity of only about 372 mAh/g duringlithiation.

Silicon, germanium, tin, and many other materials are attractive activematerials because of their high electrochemical capacity. For example,the theoretical capacity of silicon in lithium ion cells has beenestimated at about 4200 mAh/g. Yet many of these materials not beenwidely adopted in commercial batteries. One reason is the substantialchange in volume they undergo during cycling. For example, siliconswells by as much as 400% when charged to a level at or near itstheoretical capacity (Li_(4.4)Si). Volume changes of this magnitude cancause substantial stresses in active material structures resulting infractures and pulverization, loss of electrical connections within theelectrode, and capacity fading of the battery.

Conventional methods of electrode fabrication using slurries, whereslurries include high capacity active material particles and polymerbinders, typically result in electrochemical cells with poor cycle life.Most polymer binders are not sufficiently elastic to accommodate activematerial's swelling, which results in separation between polymers andactive material particles during the discharge and loss of electricalconnection between the active material particles and the currentcollector.

Overall, there is a need for improved application of high capacityactive materials in battery electrodes that minimize the drawbacksdescribed above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates three general stages a Vapor Liquid Solid (VLS)deposition process in accordance with certain embodiments.

FIG. 2 is a schematic representation of an electrode cross-sectioncontaining active materials, a substrate, and an intermediate layer inaccordance with certain embodiments.

FIG. 3 is an expanded schematic representation of a portion of anelectrode cross-section further illustrating certain details of anintermediate layer in accordance with certain embodiments.

FIG. 4 is a flow chart of a general process for fabricating an electrodecontaining an intermediate layer in accordance with certain embodiments.

FIGS. 5A-B are a top schematic view and a side schematic view of anillustrative electrode arrangement in accordance with certainembodiments.

FIGS. 6A-B are a top schematic view and a perspective schematic view ofan illustrative round wound cell in accordance with certain embodiments.

FIG. 7 is a top schematic view of an illustrative prismatic wound cellin accordance with certain embodiments.

FIGS. 8A-B are a top schematic view and a perspective schematic view ofan illustrative stack of electrodes and separator sheets in accordancewith certain embodiments.

FIG. 9 is a schematic cross-section view of an example of a wound cellin accordance with embodiments.

SUMMARY

Provided are novel electrodes for use in lithium ion batteries. Anelectrode includes one or more intermediate layers positioned between asubstrate and an electrochemically active material. Intermediate layersmay be made from chromium, titanium, tantalum, tungsten, nickel,molybdenum, lithium, as well as other materials and their combinations.An intermediate layer may protect the substrate, help to redistributecatalyst during deposition of the electrochemically active material,improve adhesion between the active material and substrate, and otherpurposes. In certain embodiments, an active material includes one ormore high capacity active materials, such as silicon, tin, andgermanium. These materials tend to swell during cycling and may loosemechanical and/or electrical connection to the substrate. A flexibleintermediate layer may compensate for swelling and provide a robustadhesion interface. Provided also are novel methods of fabricatingelectrodes containing one or more intermediate layers.

In certain embodiments, an electrode for use in a lithium ion batteryincludes a substrate, one or more intermediate layers formed on thesubstrate, and an electrochemically active material in the form ofnanostructures formed over the one or more intermediate layers andoperable for inserting and removing lithium ions during battery cycling.The electrochemically active material is in electrical communicationwith the substrate. In certain embodiments, a substrate includes one ormore of the following materials: copper, nickel, aluminum, stainlesssteel, and titanium. In the same or other embodiments, the activematerial includes one or more of the following materials: silicon, tin,germanium, a silicon-germanium combination, tin oxide, siliconoxycarbide (SiOC), and their compounds. In more specific embodiments,the active material includes silicides or, even more specifically,nickel silicides. For example, an active material may include nickelsilicide nanowires with an amorphous silicon layer formed over thenanowires. In certain embodiments, the active material nanostructuresare substrate-rooted nanowires.

In certain embodiments, one or more intermediate layers include one ormore of the following elements: chromium, titanium, tantalum, tungsten,nickel, molybdenum, iron, and lithium. A thickness of the intermediatelayers may be between about 1 nanometer and 2000 nanometers. Anelectrical resistance over a unit of surface area of the intermediatelayers may be less than about 1 Ohm-centimeter squared.

In certain embodiments, one or more intermediate layers include adiffusion barrier layer configured to shield the substrate duringformation of the electrochemically active material. In the same or otherembodiments, the intermediate layers include an adhesion layerconfigured to maintain mechanical connection between the substrate andthe electrochemically active material during battery cycling. Anintermediate layer may have a surface tension configured for depositinga catalyst layer and forming catalyst islands from the catalyst layerduring formation of the active material. In the same or otherembodiments, one or more intermediate layers are configured to separatecatalyst particles from a carrier fluid. An intermediate layer mayinclude an exposed surface having a roughness that enables distributionof a catalyst in discreet patches. An intermediate layers may have asurface condition providing nucleation sites for facilitating depositionof the electrochemically active material.

Provided also a method of manufacturing a battery electrode for use in alithium ion battery. A method may involve receiving a substrate for thebattery electrode, forming a conductive intermediate layers on thesubstrate, and depositing an electrochemically active materialcomprising nanowires on the one or more intermediate layers. Theelectrochemically active material is configured for inserting andremoving lithium ions during battery cycling. Depositing theelectrochemically active material may involve a vapor-solid-solidchemical (VLS) vapor deposition (CVD) technique. In certain embodiments,depositing the active material involves first depositing a catalyst onthe one or more intermediate layers. Two or more intermediate layers maybe deposited. In certain embodiments, an intermediate layer includes asurface condition that enhances nucleation of the active material duringthe deposition of the active material.

These and other aspects of the invention are described further belowwith reference to the figures.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Thepresent invention may be practiced without some or all of these specificdetails. In other instances, well known process operations have not beendescribed in detail to not unnecessarily obscure the present invention.While the invention will be described in conjunction with the specificembodiments, it will be understood that it is not intended to limit theinvention to the embodiments.

Introduction

Instead of binding active materials to a substrate using a polymerbinder as is conventionally done in lithium ion battery manufacturing,active materials may be attached directly to the current collectingsubstrate either during their fabrication by deposition or otherwise(thereby producing “growth rooted” active materials) or after theirfabrication (by, e.g., sintering or otherwise fusing). In certainembodiments, a surface of the substrate may need to be protected duringthe fabrication or attachment process. The reasons for needing suchprotection, as well as techniques for applying such protection aredescribed below. For now, it should be understood that the protection isprovided by one or more “intermediate layers” interposed between theelectrode substrate and the active materials. It should also beunderstood that the active material is often in the form of a smallparticles or “nanostructures,” which will be described in more detailbelow.

High capacity active materials generally experience substantial volumechange during electrochemical cycling of the cell. Such active materialsmay loose electrical and mechanical connection with the substrateleading to cell degradation. One way to address this issue is by bondingthe active materials, which may be in the form of nanostructures, to thesubstrate. In some cases, the active material attaches to the substratein a manner referred to as “substrate-rooting.” This arrangementprovides direct mechanical support and electrical communication betweenthe substrate and active materials; often this will provide ametallurgical connection (which does not necessarily mean that theconnection is lattice matched) and/or electrical coupling (and/orconnection) between the substrate and the active materials. Variousexamples of the substrate-rooted nanostructures and correspondingfabrication methods are described in U.S. patent application Ser. No.12/437,529 filed on May 7, 2009, which is incorporated by referenceherein in its entirety for purposes of describing substrate rootednanostructures.

In certain embodiments, nanostructures have one dimension that issubstantially larger than the other two. The largest dimension isreferred to as a length. Some nanostructures, especially ones with highaspect ratios, may have curved shapes. In these cases, the length of thenanostructure is the length of the representative curve. A cross-sectionis defined as a profile of a nanostructure in a plane perpendicular tothe length. Nanostructures may have many varying cross-sectional(transverse) dimensions along their lengths. Further, an active layermay have nanostructures with different cross-sections, both shapes anddimensions (e.g., tapered nanostructures). Examples of nanostructureshapes include spheres, cones, rods, wires, arcs, saddles, flakes,ellipsoids, tapes, etc.

Cross-sectional shapes are generally dependent on compositions,crystallographic structures (e.g., crystalline, poly-crystalline,amorphous), sizes, deposition process parameters, and many otherfactors. Shapes may also change during cycling. Irregularities ofcross-sectional shapes require a special dimensional characterization.For the purposes of this application, a cross-section dimension isdefined as a distance between the two most separated points on aperiphery of a cross-section that is transverse to the principaldimension, such as length. For example, a cross-section dimension of acylindrical nano-rod circle is the diameter of the circularcross-section. In certain embodiments, a cross-section dimension ofnanostructures is between about 1 nanometer and 10,000 nanometers. Inmore specific embodiments, a cross-section dimension is between about 5nanometers and 1000 nanometers, and more specifically between 10nanometers and 400 nanometers. Typically, these dimensions represent anaverage or mean across the nanostructures employed in an electrode.

In certain embodiments, nanostructures are hollow. They may be alsodescribed as tube or tube-like structures. Therefore, thecross-sectional profile of these hollow nanostructures includes voidregions surrounded by annular solid regions. An average ratio of thevoid regions to the solid regions may be between about 0.01 and 100,more specifically between about 0.01 and 10. The cross-section dimensionof the hollow nanostructures may be substantially constant along theprincipal dimension (e.g., typically the axis). Alternatively, thehollow nanostructures may be tapered along the principal dimension. Incertain embodiments, multiple hollow nanostructures may form acore-shell arrangement similar to multiwall nanotubes. Additionalexamples of hollow nanostructures are provided in U.S. patentapplication Ser. No. 12/787,138, entitled “INTERCONNECTED HOLLOWNANOSTRUCTURES CONTAINING HIGH CAPACITY ACTIVE MATERIALS FOR USE INRECHARGEABLE BATTERIES” filed on May 10, 2010, which is incorporatedherein by reference in its entirety for purposes of describing hollownanostructures.

In certain embodiments, a “nanowire” is defined as a structure that has,on average, an aspect ratio of at least about four. In certain examples,the average aspect ratio may be at least about ten, at least about onehundred, or even at least about one thousand. In some cases, the averagenanowire aspect ratio may be at least about ten thousand, and can evenreach about one hundred thousand. Nanowire active materials can undergosubstantial swelling without disrupting the overall structure of theactive layer, provide better electrical and mechanical connections withthe layer, and can be easily realized using the vapor-liquid-solid andvapor-solid template free growth methods or other templated methods.

Substrate-rooted nanostructures may be deposited on a substrate usingvarious methods. One such method is a chemical-vapor deposition (CVD)that employs a vapor-liquid-solid (VLS) phase transformation of adeposited material. This approach will be referred to herein as a “VLS”technique. Another method includes a CVD with a vapor-solid-solid (VSS)phase transformation, referred to herein as a “VSS” technique.

In various embodiments, an intermediate layer is positioned between thesubstrate and the active material to facilitate the fabrication or useof a lithium negative electrode. In one example, an intermediate layerserves to protect the substrate from reactants used to deposit theactive material. Such intermediate layer may also (or alternatively)facilitate formation of the active material by VLS or other suitableprocess. It may accomplish this by, e.g., preventing a depositioncatalyst from being contaminated by materials diffusing from thesubstrate or prevent catalyst defusing into the substrate.

A general description of a VLS process is provided here to betterunderstand certain functions and structures of intermediate layers andother components of the electrode, in accordance with certainembodiments. VLS is a mechanism for the growth of one-dimensionalstructures, such as nanowires, from CVD. The VLS process introduces acatalytic liquid alloy phase, which can rapidly adsorb a precursor vaporto super-saturation levels, and thereby facilitate crystal growth at theliquid-solid interface.

FIG. 1 illustrates three general stages of a typical VLS depositionprocess in accordance with certain embodiments. During the initial stage100, discrete catalyst islands 104 are formed on the surface of thesubstrate 102. The surface typically has an intermediate layer, notshown, which is described below in more detail.

A substrate may be a metallic foil, an open structure substrate (e.g.,mesh, foam), a composite that include structural and conductivematerials, and other forms. Substrate materials for electrodes used invarious lithium ion cells may include copper and/or copper dendritecoated metal oxides, stainless steel, titanium, aluminum, nickel (alsoused as a diffusion barrier), chromium, tungsten, metal nitrides, metalcarbides, metal oxides, carbon, carbon fiber, graphite, graphene, carbonmesh, conductive polymers, or combinations of above includingmulti-layer structures. It will be understood by one having ordinaryskills in the art that selection of the materials also depends onelectrochemical potentials of the materials. The substrate material maybe formed as a foil, films, mesh, laminate, wires, tubes, particles,multi-layer structure, or any other suitable configurations. Forexample, the substrate 102 may be a stainless steel foil havingthickness of between about 1 micrometer and 50 micrometers. In otherembodiments, the substrate 102 is a copper foil with thickness ofbetween about 5 micrometers and 50 micrometers. Certain substrateexamples are described in U.S. patent application Ser. No. 12/437,529filed on May 7, 2009 and U.S. patent application Attorney Docket No.AMPRP005P filed herewith, which are incorporated by reference herein intheir entirety for purposes of describing substrates.

The catalyst islands 104 may be formed by first depositing a continuouslayer containing catalyst and then removing parts of the layer (by,e.g., using lithographic etching, ablating) or breaking the continuouslayer by thermal annealing. An intermediate layer may be used to protectthe substrate during this removal. In other embodiments, a continuous orpartial layer containing catalyst (typically, a eutectic alloycontaining metallic catalyst, such as gold) is heated, which leads toformation of discrete droplets due to the surface tension. Anintermediate layer may be used to change the surface properties of thesubstrate, to form a eutectic alloy with catalyst containing material,prevent catalyst losses into the substrate (e.g., a gold catalyst overthe copper substrate), and other purposes. Some properties that mayimpact formation of discontinuous catalyst islands include surfaceroughness, grain structure and porosity, magnetic orientation, andelectronic structure.

An intermediate layer may contain a catalyst, which may be, e.g.,plated, sputtered, and/or evaporated on the intermediate layer. Incertain embodiments, materials of the intermediate layer and catalystmay be deposited together and then subjected to phase separation tocontrol distribution of the materials in this combined layer. Nanoand/or micro crystals may occur near or at the exposed surface of theintermediate layer. The size of the crystals may be controlled duringthe deposition process. For example, the power level, chamber pressure,and/or temperature may be controlled during sputtering. If plating isused for deposition of the materials, then plating currents and bathcomposition can be controlled. Furthermore, certain post depositiontreatment parameters (e.g., temperature and/or duration for annealing)may be controlled. As resulting distribution of the catalyst on thesurface effect density and size of nanowires in some of theseembodiments.

In certain embodiments, roughness of the intermediate layer andformation of catalyst islands is established by chemical etching.Etchant may be introduced after the deposition of the intermediate lateror during such deposition (e.g., close the end of the deposition) andreact with the intermediate layer to create rougher surface and formcatalyst islands.

Further, an intermediate layer may have portions with different chemicalor physical properties (e.g., polarization, binding sites, magneticproperties), which can be used to distribute catalysts particles or toform islands during a deposition process.

In certain embodiments, catalyst containing materials may be depositedon a substrate as discrete catalyst islands without first forming acontinuous layer. For example, a slurry solution with catalyst particlesand/or a catalyst suspension (e.g., a colloid suspension) may be used tocoat the substrate surface. The slurry is then dried to form catalystislands. Certain details of these embodiments are described in U.S.patent application Ser. No. 11/103,642 filed on Apr. 12, 2005, which isincorporated herein in its entirety for purposes of describing processexamples of forming catalyst islands. In these embodiments, intermediatelayers mat be used to provide desirable surface properties for slurryflow and drying and protect the substrate from the slurry. In otherembodiments, catalytic materials are embedded onto the intermediatelayer such that only a portion of the catalyst material is exposed.Other methods for depositing a catalyst include electroless depositionand mixing a salt precursor with catalyst elements followed by heatingor annealing the mixtures with a presence of hydrogen.

Materials suitable for the catalyst include any materials capable ofreacting and forming a compound with the process gas in, for example,VLS or VSS types of deposition processes. Examples include gold, nickel,cobalt, aluminum, copper, gallium, indium, silver, titanium, carbon,carbides, alloys, and mixtures of thereof. Catalysts can be depositedusing thermal evaporation, sputtering, electroplating, and filtrationmethods, etc. Depending on the deposition condition, either a continuousfilm or discrete catalyst islands form on the intermediate layer

In certain embodiments, deposition using evaporation or sputtering ofcatalyst over a rough surface of the substrate or an intermediate layercreates shadowing effects, which results in clustered deposition. Thismay eliminate a need for a separate post-deposition treatment to createcatalyst islands. Further, platting on rough surfaces may result inpreferential deposition on the extending tips of the rough surfacestructure caused by uneven field distribution.

In certain embodiments, depositing small amounts of certain catalystmaterials on certain surfaces (e.g. gold on silicon oxides and/orsilicon) do not form a homogenous single atomic layer and insteaddeposit in clusters. Without being restricted to any particular theory,it is believed that thermodynamic driving forces of the surface tensioneffect such distribution). Clustering may be controlled by controllingdeposition process conditions, such as temperature and deposition rate.

In certain embodiments, plating on partially oxidized surfaces orsurfaces with a porous template (e.g., porous polymers) on the top of it(porous polymer) is used to form catalyst islands. For example, asubstrate or an intermediate layer may be partially oxidized by heatingat ambient conditions or by introducing an oxidizing agent into thedeposition chamber. Alternatively or in addition to this method, asurface may be then coated with a polymer that forms a porous structureduring the deposition or during subsequent treatment (e.g., heating).

In certain embodiments, pulse plating of the catalyst may results incatalyst islands formed on the surfaced. For example, relatively shortpulse duration can be used to form a discontinuous film. The duration ofthe pulse depends on the plating bath configuration, plating currents,plating bath composition, deposition surface materials and geometry(e.g., surface roughness), and other process parameters.

In certain embodiments, a partial electrochemical dissolution of thecatalyst layer is used to form islands of the metal. For example, apulsed current, a template, roughening or oxidizing the surface may beused to establish selective dissolution.

Various criteria may be taken into account in selecting catalystmaterials. Such criteria include melting and eutectic points withnanostructure materials, wetting properties such as surface tension onthe intermediate layer (to form catalyst islands upon melting), bulkdiffusivity in the intermediate layer, impact on electrochemical andelectrical properties of deposited nanostructures, and others. Forexample, aluminum has lower diffusivity in crystalline silicon than goldbut also makes a eutectic with silicon, though at higher temperaturethan gold (about 577° C.). Copper, in turn, diffuses very fast insilicon but has even higher eutectic (about 802° C.). Copper may be usedto grow silicon nanowires in a Vapor-Solid-Solid mode. Further, galliumhas both low melting temperature and low diffusivity in silicon incomparison to gold.

In the next stage 110 of the VLS process, one or more precursor gases106 are provided to the surface of the substrate 102 containing catalystislands 104 a. These precursor gases can decompose or otherwise react toform electrochemically active materials, such as silicon, germanium,silicon-germanium alloys (SiGe), silicon oxycarbide (SiOC), tin, tinoxide, titanium oxide, carbon, a variety of metal hydrides (e.g., MgH₂),silicides, phosphides, carbides and nitrides, that later formnanostructures 112. The precursor gases 106 react at the surface of thecatalyst islands 104 releasing certain materials 108 that are adsorbedby the islands 104 a and other materials 119 and then released to theenvironment. This process is sometimes referred to as a dissociativechemisorption. For example, silane (SiH₄) decomposes at hightemperatures or with an assist of plasma to produce silicon, silaneradical, and hydrogen. Deposited silicon or silicon-containing materialthen diffuses into the catalyst islands and form alloys with thecatalysts. Another example is chloride based silane such as dichloride,trichloride, and tetrachloride silane. Chlorosilanes (H_(x)SiCl_(4-x))may react with hydrogen (H₂) on the surface of a gold containingcatalyst island and release silicon (Si) into the islands and hydrogenchloride (HCl) into environment of the processing chamber.

As the dissociative chemisorption process continues, the catalystislands 104 a increase the concentration of the adsorbed materials 108until it reaches the saturation level. At this point, shown in the nextstage 120, further adsorption of the material 108 causes precipitationof this material at the substrate interface leading to formation of asolid nanostructure 112. This nanostructure 112 contains activematerials and, in certain embodiments, other materials configured toenhance conductivity (e.g., dopants), structural integrity, adhesion tothe substrate, and other properties of the nanostructures. Thenanostructures may be functionalized during or after deposition, e.g.,forming core-shell arrangement with other materials, pre-loaded withlithium.

In these and other embodiments, an intermediate layer may be used toprotect the substrate 102 from interacting with materials in thecatalyst islands 104 a and, as well, the nanostructures 112, precursors106 and released reaction products 119 during the VLS-type depositionprocess, and during functionalization. For example, a gold-containingcatalyst may be used to deposit silicon nanowires. However, depositinggold on a copper substrate leads to formation of a gold-copper alloy,which may negatively impact the catalytic effect and require more goldto be deposited. Further, copper may form silicides when exposed tosilane, silicon tetrachloride, or other silicon containing precursorgas. Copper silicides are generally not desirable in silicon basedelectrodes due to it poor mechanical and undesired electrochemicalproperties.

Providing an intermediate layer allows using various substrate materialsthat otherwise would react or with precursor gases (e.g., silane) orform alloys with the deposited materials during or after the deposition.For example, depositing silicon nanowires directly on a copper or nickelsubstrate may result in formation of undesirable silicides. Anintermediate layer serves as a barrier during deposition and preventscontact between such substrates and precursors gases. As a result, anumber of possible material alternatives for substrates is greatlyincreased.

In certain embodiments, pre-fabricated nanostructures are bonded (e.g.,fused or sintered) to the substrate surface using a combination of heatand pressure or other techniques. An intermediate layer may enhance thebonding formed by these techniques. In other embodiments,substrate-rooted nanostructures are formed by depositing a bulk layer ofthe active material onto the substrate and then selectively etchingparts of the layer forming substrate-rooted nanostructures. A substratemay need to be protected from etchants in this embodiments, e.g., usingan intermediate barrier layer.

In certain other embodiments, high capacity materials may be bound tothe substrate using polymeric binders. An intermediate layer depositedon the substrate may allow using binders to accommodate for excessiveswelling of high capacity materials yet to maintain a sufficientelectrical and mechanical communication with the substrate. For example,an intermediate layer may be used to increase substrate surfaceroughness. In other embodiments, an intermediate layer includesfunctional groups on its surface that provide better adhesion of thepolymer to the substrate. It should be understood that embodimentsrelying on binders will not typically provide a substrate-rootedstructure nor will they provide a metallurgical bond between thesubstrate and the active materials nanostructures.

In certain embodiments, pre-synthesized (e.g., preformed) nanoparticlesare deposited on the substrate followed by thermal annealing steps toform metallurgical connections between the nanoparticles and substrates.An intermediate layer may be used to assist during this bond formationor other parts of the overall process.

Structure and Materials of Intermediate Layer

An intermediate layer may be used as a diffusion barrier. For example,an intermediate layer may prevent substrate materials from diffusinginto (and thereby degrading the performance of) catalysts used to growactive material nanostructures. Additionally, in some cases, theintermediate layer may prevent interaction between the substrate andactive material precursors and/or other reagents used during activematerial fabrication and other processing operations. Further, theintermediate layer may enhance adhesion of the active material to thesubstrate, especially when nanostructures undergo substantial volumechange during cycling. Still further, an intermediate layer may provide,e.g., an epitaxial or chemical-bond connection between the substrate andactive material nanostructures (to address a lattice mismatch and reducestrain), and/or a thermal expansion coefficient that allows electrodesub-assemblies to be brought from processing temperatures (e.g.,deposition temperature, post-deposition treatment temperatures) to theroom temperature without causing fractures at the substrate-activematerial interface, and be electrically conductive. An intermediatelayer could facilitate or accelerate nanowire growth since surfaceroughness and wetability between the intermediate layer and catalystislands can be optimized by choosing different deposition process anddifferent intermediate materials. An intermediate layer may be also usedto promote mechanical integrity during a roll to roll or other method offabrication (e.g., prevent deformation because of a high temperature,high tension environment).

Selection of materials for an intermediate layer depends on substratematerials, active materials, contact/attachment conditions, targetedfunctionality of the intermediate layer, and other parameters. Examplesof intermediate layer materials include refractory metals, such astungsten, molybdenum, niobium, tantalum, rhenium, tungsten nitride,tungsten carbide, titanium, titanium oxide, titanium nitride, titaniumcarbide, zirconium, zirconium nitride, tantalum, tantalum nitride,cobalt, ruthenium, indium oxide, cadmium, hafnium, tellurium, telluriumoxide, tellurium nitride, chromium, iron, chromium oxide, atitanium-tungsten combination, an iron-tungsten combination, acobalt-tungsten combination, molybdenum, nickel, lithium and others. Athickness of the intermediate layer may be between about 1 nanometer and5 micrometers, more specifically between about 5 nanometers and 1micrometer, even more specifically between about 25 nanometers and 100nanometers. Introducing certain materials into the layer, such as coppernickel, chromium, and titanium may improve adhesion of depositednanostructures to the substrate surface. The thickness generally dependson functionality required from the layer and corresponding properties ofthe materials included in the layer. In certain embodiments, theintermediate layer has a contact resistance per unit surface area of thelayer that is less than about 10 Ohm-centimeter squared or in morespecific embodiments less about 5 Ohm-centimeter squared. A resistanceover a unit of surface area is defined as a resistivity of theintermediate layer materials multiplied by a thickness of the layer.

In certain embodiments, an intermediate layer includes tungsten having athickness of between about 150 nanometers and 250 nanometers. Tungstendoes not form alloys with many materials that can be used as a catalystto deposit high capacity nanostructured materials. In other embodiments,a composite intermediate layer is used containing a sub-layer oftungsten containing material (e.g., between about 150 nanometer and 250nanometer thick) and a sub-layer of titanium containing material (e.g.,between about 1 nanometers and 50 nanometers thick). The titaniumsub-layer may be used to enhance adhesion of the intermediate layer tothe substrate. Intermediate layers described above may be used withcopper and nickel substrates.

In certain embodiments, an intermediate layer includes chromium and hasa thickness of between about 500 nanometers and 1,500 nanometers. Whilechromium forms an alloys with gold (and possible can not be used withthis type of catalyst), it can be successfully used with other catalystand be deposited over copper, nickel, and silver substrate layers.

In certain embodiments, the electrode includes multiple intermediatelayers that form a stack. Each of these layers may contain the same ordifferent materials. A stack of the intermediate layers may also bereferred to as a “barrier system”. For examples, FIG. 2 illustrates aschematic cross-section of an electrode 200 with a stack 204 that ispositioned between the substrate 202 and the active materialnanostructures 206. FIG. 3 illustrates an expanded view of an electrodeportion 210 with a stack 204 containing three layers 208, 210, and 212.It should be understood that a stack may include any number ofintermediate layers (e.g., 1, 2, 3, 4, 5, or 6). A number of layers maydepend on materials used, deposition techniques, and targetedfunctionality. In certain embodiments, a composite intermediate layer isused, for example, as a combination of an adhesion layer and a diffusionbarrier, as a combination of a diffusion barrier and a nucleationsurface layer, as a combination of an adhesion layer, a diffusionbarrier, and a nucleation surface layer, and in a case where twomaterials provide better diffusion barrier than just one (e.g.,synergistic diffusion barrier effects).

In certain embodiments, one layer in a stack may be used to improveadhesion of the nanostructures to the substrate. In more specificexamples, one layer (e.g., layer 207 in FIG. 3) may be used to improveadhesion of the substrate to the stack, while another layer (e.g., layer209 in FIG. 3) may be used to improve adhesion of the nanostructures tothe stack. Examples of materials for such layers include chromium,titanium, tungsten, tantalum, nickel, and molybdenum. An adhesion layermay be chosen to accommodate substantial swelling of the nanostructurebase, while the substrate remains substantially static.

A layer, for example layer 208 in FIG. 3, may be used as a diffusionbarrier. This layer may prevent the substrate from interacting withcatalyst islands, precursors, and reaction products. In certainembodiments, an intermediate layer (e.g., layer 209 in FIG. 3), may beused to assist in formation of catalyst islands during the VLS and/orVSS deposition process. For example, a layer may be used to modifysurface properties of the substrate to provide adequate surface tensionso that the catalyst islands are agglomerated or aggregated. In otherembodiments, the layer may be used to prevent substrate damage duringlithographic etching, ablation, and other methods of forming catalystislands.

In certain embodiments, an intermediate layer or a portion of theintermediate layer (e.g., a top sub-layer, such as layer 209 in FIG. 3)has a surface condition that facilitates nucleation of thenanostructures deposited onto the substrate, or more specifically ontothe intermediate layer of the substrate. The surface condition may becreated as a part of an overall catalyst island formation operation or aseparate operation. As understood, in VLS or VSS deposition methods asolution containing an active material (or a precursor thereof)precipitates a solid phase containing the active material when theconcentration of the active material in the solution reaches a certainhigh level. Initiation of this precipitation can be controlled, to acertain degree, by controlling the surface properties of theintermediate layer in the contact with the solution. Examples of theseproperties include surface roughness, surface polarization, surfacetension, and morphology of the surface materials (e.g., crystalline,amorphous, lattice size and orientation). These properties can becontrolled by selecting certain materials for an intermediate layer (orportions thereof). Examples of such materials include chrome, tungsten,nickel, molybdenum, iron, as well as mixtures and alloys containing oneor more of these materials. Further, these properties can be controlledby using certain deposition methods and controlling process parametersduring the deposition. Examples of deposition methods includesputtering, electrodeposition (e.g., electroless deposition,pulse-plating), evaporation, chemical vapor deposition (CVD), physicalvapor deposition (PVD), and atomic layer deposition (ALD). In certainembodiments, a post-deposition treatment, such as back-plating,electro-etching, resputtering, CVD, annealing, plasma etching, andoxidation, is used to further control properties of the intermediatelayer. While the specific surface condition(s) chosen to facilitatenucleation will depend on the composition of the intermediate layer, theactive material, and the catalyst (if any), generally a surface having asurface roughness of at least about 0.01 μm R, more specifically atleast about 0.05 μm R, or at least about 0.1 μm R. Frequently, surfaceconditions that match properties of the active material will bepreferred. For example, substantially similar lattice constants,polarizations, surface tensions, etc. for the intermediate layer andactive material are preferred.

Process

FIG. 4 illustrates a flow chart of a general process 400 for depositingactive material on a substrate with one or more intermediate layerpositioned between the substrate and the active material(s). The process400 may start with providing a substrate (block 402). In certainembodiments, a substrate may be provided into a processing chamber, sucha CVD apparatus, in a roll-to-roll format. A deposition area of thesubstrate is generally preheated to a predetermined temperatureestablished by deposition conditions of the intermediate layer and/oractive materials.

The process 400 may continue with depositing one or more intermediatelayers (block 404). In certain embodiments, intermediate layer materialsare deposited using a Physical Vapor Deposition, a Chemical VaporDeposition, electrodeposition, or any other suitable depositiontechnique. For example, a layer of titanium and/or titanium nitride maybe deposited using a sputtering target containing titanium as well asevaporation, sputtering, plating, laser ablation, Atomic LayerDeposition, and Chemical Vapor Deposition. This deposition operation(block 404) may be followed by one or more post deposition treatmentoperations, such as back-plating/electro-etching, resputtering, CVDtreatment, annealing, plasma etching, and oxidation. For example,surface properties of the intermediate layer may need to be controlledto allow formation of the catalyst islands and/or nucleation of theactive material during active material deposition operation 406. Incertain embodiments, intermediate layer deposition operation 404 and/orpost-deposition treatment may be repeated a number of times to build anintermediate layer stack as, for example, shown in FIG. 4.

The process 400 may continue with depositing active materials (block406). Details of some embodiments of this operation are described inU.S. patent application Ser. No. 12/437,529 filed on May 7, 2009, whichis incorporated by reference herein in its entirety for purposes ofdescribing an operation for depositing active materials.

In certain embodiments, particularly those involving a VLS or VSSprocess, the deposition operation 406 starts with depositing catalystislands on the substrate surface. In addition to single materialcatalyst embodiments, catalyst islands may include two or more materials(e.g., binary catalysts, tertiary catalysts, etc.). Besides modifyingcatalytic functions, a combination of catalyst may lead to changes ineutectic properties, rheological properties (e.g., viscosity, surfacetension), and other described above.

In certain embodiments, deposition processes other than a VLS may beused to deposit active materials in operation 406. Certain examples aredescribed above.

It should be noted that the above mentioned operations could beimplemented on a single apparatus or a series of apparatus such thatoperations are performed soon after completion of the previousoperation. For example, an apparatus may include one or more sputteringstations for adding intermediate layer and catalyst materials and one ormore CVD stations for depositing active material nanostructures onto themoving web. In other embodiments, different apparatuses may be used forone or more of theses. A period of time may pass before two sequentialoperations, in which case, a partially manufactured electrodes may needto be protected from the storage environment by adding a protectivelayer.

Sub-Assembly: Electrodes with Separators

Two common arrangements of the electrodes in lithium ion cells are woundand stacked. One goal is to position and align the surfaces of activelayers of the two electrodes surfaces as close as possible withoutcausing an electrical short. Close positioning allows lithium ions totravel more rapidly and more directly between the two electrodes leadingto better performance.

FIG. 5A illustrates a side view of an aligned stack including a positiveelectrode 502, a negative electrode 504, and two sheets of the separator506 a and 506 b in accordance with certain embodiments. The positiveelectrode 502 may have a positive active layer 502 a and a substrateportion 502 b not coated with a positive active material (but mayinclude an intermediate layer coating), i.e., an uncoated portion.Similarly, the negative electrode 504 may have a negative active layer504 a and a negative substrate portion 504 b not coated with a negativeactive material (but may include an intermediate layer coating), i.e.,an uncoated portion. In many embodiments, the exposed area of thenegative active layer 504 a is slightly larger that the exposed area ofthe positive active layer 502 a to ensure trapping of the lithium ionsreleased from the positive active layer 502 a by intercalation materialof the negative active layer 504 a. In one embodiment, the negativeactive layer 504 a extends at least between about 0.25 and 5 mm beyondthe positive active layer 502 a in one or more directions (typically alldirections). In a more specific embodiment, the negative layer extendsbeyond the positive layer by between about 1 and 2 mm in one or moredirections. In certain embodiments, the edges of the separator sheets506 a and 506 b extend beyond the outer edges of at least the negativeactive layer 504 a to provide electronic insulation of the electrodefrom the other battery components. The positive uncoated portion 502 bmay be used for connecting to the positive terminal and may extendbeyond negative electrode 504 and/or the separator sheets 506 a and 506b. Likewise, the negative uncoated portion 504 b may be used forconnecting to the negative terminal and may extend beyond positiveelectrode 502 and/or the separator sheets 506 a and 506 b.

FIG. 5B illustrates a top view of the aligned stack. The positiveelectrode 502 is shown with two positive active layers 512 a and 512 bon opposite sides of the flat positive current collector 502 b.Similarly, the negative electrode 504 is shown with two negative activelayer 514 a and 514 b on opposite sides of the flat negative currentcollector. Any gaps between the positive active layer 512 a, itscorresponding separator sheet 506 a, and the corresponding negativeactive layer 514 a are usually minimal to non-existent, especially afterthe first cycle of the cell. The electrodes and the separators areeither tightly would together in a jelly roll or are positioned in astack that is then inserted into a tight case. The electrodes and theseparator tend to swell inside the case after the electrolyte isintroduced and the first cycles remove any gaps or dry areas as lithiumions cycle the two electrodes and through the separator.

A wound design is a common arrangement. Long and narrow electrodes arewound together with two sheets of separator into a sub-assembly,sometimes referred to as a jellyroll, shaped and sized according to theinternal dimensions of a curved, often cylindrical, case. FIG. 6A showsa top view of a jelly roll comprising a positive electrode 606 and anegative electrode 604. The white spaces between the electrodesrepresent the separator sheets. The jelly roll is inserted into a case602. In some embodiments, the jellyroll may have a mandrel 608 insertedin the center that establishes an initial winding diameter and preventsthe inner winds from occupying the center axial region. The mandrel 608may be made of conductive material, and, in some embodiments, it may bea part of a cell terminal. FIG. 6B presents a perspective view of thejelly roll with a positive tab 612 and a negative tab 614 extending fromthe jelly roll. The tabs may be welded to the uncoated portions of theelectrode substrates.

The length and width of the electrodes depend on the overall dimensionsof the cell and thicknesses of active layers and current collector. Forexample, a conventional 18650 cell with 18 mm diameter and 65 mm lengthmay have electrodes that are between about 300 and 1000 mm long. Shorterelectrodes corresponding to low rate/higher capacity applications arethicker and have fewer winds.

A cylindrical design may be desirable for some lithium ion cells becausethe electrodes swell during cycling and exert pressure on the casing. Around casing may be made sufficiently thin and still maintain sufficientpressure. Prismatic cells may be similarly wound, but their case maybend along the longer sides from the internal pressure. Moreover, thepressure may not be even within different parts of the cells and thecorners of the prismatic cell may be left empty. Empty pockets may notbe desirable within the lithium ions cells because electrodes tend to beunevenly pushed into these pockets during electrode swelling. Moreover,the electrolyte may aggregate and leave dry areas between the electrodesin the pockets negative effecting lithium ion transport between theelectrodes. Nevertheless, in certain applications, such as thosedictated by rectangular form factors, prismatic cells are appropriate.In some embodiments, prismatic cells employ stacks rectangularelectrodes and separator sheets to avoid some of the difficultiesencountered with wound prismatic cells.

FIG. 7 illustrates a top view of a wound prismatic jellyroll. The jellyroll comprises a positive electrode 704 and a negative electrode 706.The white space between the electrodes is representative of theseparator sheets. The jelly roll is inserted into a rectangularprismatic case. Unlike cylindrical jellyrolls shown in FIGS. 6A and 6B,the winding of the prismatic jellyroll starts with a flat extendedsection in the middle of the jelly roll. In one embodiment, the jellyroll may include a mandrel (not shown) in the middle of the jellyrollonto which the electrodes and separator are wound.

FIG. 8A illustrates a side view of a stacked cell including a pluralityof sets (801 a, 801 b, and 801 c) of alternating positive and negativeelectrodes and a separator in between the electrodes. One advantage of astacked cell is that its stack can be made to almost any shape, and isparticularly suitable for prismatic cells. However, such cell typicallyrequires multiple sets of positive and negative electrodes and a morecomplicated alignment of the electrodes. The current collector tabstypically extend from each electrode and connected to an overall currentcollector leading to the cell terminal.

Housing

FIG. 9 illustrates a cross-section view of the wound cylindrical cell inaccordance with one embodiment. A jelly roll comprises a spirally woundpositive electrode 902, a negative electrode 904, and two sheets of theseparator 906. The jelly roll is inserted into a cell case 916, and acap 918 and gasket 920 are used to seal the cell. In some cases, cap 912or case 916 includes a safety device. For example, a safety vent orburst valve may be employed to break open if excessive pressure buildsup in the battery. Also, a positive thermal coefficient (PTC) device maybe incorporated into the conductive pathway of cap 918 to reduce thedamage that might result if the cell suffered a short circuit. Theexternal surface of the cap 918 may used as the positive terminal, whilethe external surface of the cell case 916 may serve as the negativeterminal. In an alternative embodiment, the polarity of the battery isreversed and the external surface of the cap 918 is used as the negativeterminal, while the external surface of the cell case 916 serves as thepositive terminal. Tabs 908 and 910 may be used to establish aconnection between the positive and negative electrodes and thecorresponding terminals. Appropriate insulating gaskets 914 and 912 maybe inserted to prevent the possibility of internal shorting. Forexample, a Kapton™ film may used for internal insulation. Duringfabrication, the cap 918 may be crimped to the case 916 in order to sealthe cell. However prior to this operation, electrolyte (not shown) isadded to fill the porous spaces of the jelly roll.

A rigid case is typically required for lithium ion cells, while lithiumpolymer cells may be packed into a flexible, foil-type (polymerlaminate) case. A variety of materials can be chosen for the case. Forlithium-ion batteries, Ti-6-4, other Ti alloys, Al, Al alloys, and 300series stainless steels may be suitable for the positive conductive caseportions and end caps, and commercially pure Ti, Ti alloys, Cu, Al, Alalloys, Ni, Pb, and stainless steels may be suitable for the negativeconductive case portions and end caps.

CONCLUSION

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications may be practiced within the scope of theappended claims. It should be noted that there are many alternative waysof implementing the processes, systems, and apparatus of the presentinvention. Accordingly, the present embodiments are to be considered asillustrative and not restrictive, and the invention is not to be limitedto the details given herein.

1. An electrode for use in a lithium ion battery, the electrodecomprising: a substrate; one or more intermediate layers formed on thesubstrate; and an electrochemically active material in the form ofnanostructures formed over the one or more intermediate layers andoperable for inserting and removing lithium ions during battery cycling,wherein the electrochemically active material is in electricalcommunication with the substrate.
 2. The electrode of claim 1, whereinthe substrate comprises one or more materials selected from the groupconsisting of copper, nickel, aluminum, stainless steel, and titanium.3. The electrode of claim 1, wherein the active material comprises oneor more materials selected from the group consisting of silicon, tin,germanium, a silicon-germanium combination, tin oxide, siliconoxycarbide (SiOC), and their compounds.
 4. The electrode of claim 3,wherein the active material comprises silicides.
 5. The electrode ofclaim 4, wherein the active material comprises nickel silicides.
 6. Theelectrode of claim 1, wherein at least one of the one or moreintermediate layers comprises one or more elements selected from thegroup consisting of chromium, titanium, tantalum, tungsten, nickel,molybdenum, iron, and lithium.
 7. The electrode of claim 1, wherein athickness of the one or more intermediate layers is between about 1nanometer and 2000 nanometers.
 8. The electrode of claim 1, wherein anelectrical resistance over a unit of surface area of the one or moreintermediate layers is less than about 1 Ohm-centimeter squared.
 9. Theelectrode of claim 1, wherein the nanostructures comprisesubstrate-rooted nanowires.
 10. The electrode of claim 1, wherein theone or more intermediate layers comprise a diffusion barrier layerconfigured to shield the substrate during formation of theelectrochemically active material.
 11. The electrode of claim 1, whereinthe one or more intermediate layers comprise an adhesion layerconfigured to maintain mechanical connection between the substrate andthe electrochemically active material during battery cycling.
 12. Theelectrode of claim 1, wherein the one or more intermediate layers has asurface tension configured for depositing a catalyst layer and formingcatalyst islands from the catalyst layer during formation of the activematerial.
 13. The electrode of claim 1, wherein the one or moreintermediate layers are configured to separate catalyst particles from acarrier fluid.
 14. The electrode of claim 1, wherein the one or moreintermediate layers comprise an exposed surface having a roughness thatenables distribution of a catalyst in discreet patches.
 15. Theelectrode of claim 1, wherein the one or more intermediate layerscomprise a surface condition providing nucleation sites for facilitatingdeposition of the electrochemically active material.
 16. A method ofmanufacturing a battery electrode for use in a lithium ion battery, themethod comprising: receiving a substrate for the battery electrode;forming a conductive intermediate layers on the substrate; anddepositing an electrochemically active material comprising nanowires onthe one or more intermediate layers, wherein the electrochemicallyactive material is configured for inserting and removing lithium ionsduring battery cycling.
 17. The method of claim 16, wherein depositingthe electrochemically active material comprises a vapor-solid-solidchemical (VLS) vapor deposition (CVD) process.
 18. The method as in oneof claims 16, wherein depositing the active material includes depositinga catalyst on the one or more intermediate layers.
 19. The method as inone of claims 16, wherein the formation of the conductive intermediatelayers comprises depositing at least two intermediate layers.
 20. Themethod as in one of claims 16, wherein the intermediate layer comprisesa surface condition that enhances nucleation of the active materialduring the deposition of the active material.